Process for the purification and recovery of acetonitrile

ABSTRACT

A method for the purification of crude acetonitrile comprising distilling the crude acetonitrile in a first fractional distillation column at below atmospheric pressure, withdrawing a first side draw fraction comprising acetonitrile, distilling the first side draw fraction in a second fractional distillation column at super atmospheric pressure, and withdrawing from the second distillation a second side draw fraction comprising purified acetonitrile.

This application claims the benefit of U.S. Provisional Application No.60/218,865 filed Jul. 18, 2000.

BACKGROUND OF THE INVENTION

This invention is an improved process for the production ofacetonitrile. More particularly, this invention is an improved processfor the purification of crude acetonitrile produced, for example, as abyproduct during the formation of acrylonitrile by the ammoxidation ofan olefin such as propylene or the ammoxidation of propane. Thisinvention is also a process for the purification of recycled and wastesolvent acetonitrile.

Acetonitrile is a solvent used in a variety of industrial chemicalprocesses and is a solvent of choice for use in high performance liquidchromatography, usually in combination with one or more solvents such aswater or an alcohol. Acetonitrile is also used as a raw material for thepreparation of other chemicals used in the chemical and pharmaceuticalindustry. Pure acetonitrile is desirable for such processes and otheruses.

An important source of acetonitrile is a byproduct stream producedduring the manufacture of acrylonitrile by the catalytic ammoxidation ofpropylene such as the process disclosed in U.S. Pat. No. 5,093,299. Thecatalytic ammoxidation of propylene is a major industrial process andthe byproduct stream containing acetonitrile is produced worldwide inhundreds of millions of pounds per year. Although the amounts may vary,a typical example of this byproduct stream contains approximately 50%acetonitrile, 40% water, and smaller amounts of hydrogen cyanide (HCN),acrylonitrile, and other organic materials such as oxazole, allylalcohol, acetone, and propionitrile. Prior processes for separating thedesired acetonitrile from the other components in the byproduct mixture,particularly from water, are not simple. For example, in the processdisclosed in U.S. Pat. No. 4,362,603, three different distillationprocedures were employed. The first is a high pressure fractionaldistillation to remove lights and heavies, the second is a low pressuredistillation to form an overhead of a water-acetonitrile azeotrope and aheavy fraction which includes water, and the third is a high pressuredistillation designed to produce a middle cut or side-draw of relativelypure acetonitrile, a water-acetonitrile overhead and a bottoms or heavyfraction. While this process produces relatively pure acetonitrile, itrequires the use of three distillation steps and the attendant recyclestreams. The art needs a simpler process for the purification of crudeacetonitrile, particularly acetonitrile that is mixed with water orother solvents. The present invention provides such an improved process.

SUMMARY OF THE INVENTION

This invention is a method for the purification of crude or impureacetonitrile comprising distilling the acetonitrile in a fractionaldistillation column at below atmospheric pressure, withdrawing a firstside draw fraction comprising acetonitrile, distilling the first sidedraw fraction in a second fractional distillation column at superatmospheric pressure, and withdrawing from the second distillation asecond side draw fraction comprising purified acetonitrile.

The acetonitrile used in the method of this invention is preferablyacetonitrile produced as a byproduct during the ammoxidation ofpropylene or other suitable hydrocarbon such as propane. However, themethod of this invention can be used to purify any source ofacetonitrile including, for example, recycle or waste acetonitrile. Suchrecycle or waste acetonitrile may be acetonitrile that has been used asa solvent in other manufacturing processes, or as a solvent for highpressure liquid chromatography and may contain as impurities variousamounts of water and one or more organic impurities.

BRIEF DESCRIPTION OF THE FIGURE

The FIGURE is a schematic representation of one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In the method of this invention, a source of crude acetonitrile istreated to produce purified acetonitrile. Purified acetonitrile as usedherein means that the acetonitrile is purer, that is, in a greaterconcentration, than the crude acetonitrile used as the starting materialfor the purification method. Preferably, purified acetonitrile meansacetonitrile that is at least about 95 percent by weight pure, morepreferably at least about 98 percent by weight pure and most preferablyat least about 99 percent by weight pure. The method of this inventioncan be used to prepare acetonitrile that is at least about 99.5 percentby weight pure, more preferably at least 99.99 percent by weight pure.

The crude acetonitrile used in the method of this invention can comefrom any source. However, the method of this invention is most suitablefor purifying acetonitrile that is made as a byproduct during thecatalytic ammoxidation of a hydrocarbon feed such as propylene orpropane by reacting a mixture of, for example, propylene, ammonia and anoxygen-containing gas in the presence of a suitable catalyst. Theoxygen-containing gas may, for example, be air, air enriched with pureoxygen gas, or some other form of molecular oxygen. The catalyst issuitably one of a number of catalysts known in the art for theammoxidation of hydrocarbons such as propylene. Preferable catalysts arethose that can operate under the usual molar ratio of air to propyleneof at least about 8.5:1. Suitable catalysts are disclosed for example inU.S. Pat. Nos. 3,642,930; 4,485,079; 3,911,089; 4,873,215; 4,877,764;and Japanese Patent Application Nos. 74-87474 and 78-352322.

In such ammoxidation processes, acrylonitrile is produced as the majorproduct. However, smaller amounts of other products such as HCN,acetonitrile, oxazole, allyl alcohol, acetone, and propionitrile arecoproduced. Water is also formed. A condensed organic product mixturefrom the catalytic ammoxidation of propylene typically contains about 10to about 13 weight percent acrylonitrile, about 16 to about 19 weightpercent water, about 0.9 to about 1.5 weight percent HCN, about 0.2 toabout 0.4 weight percent acetonitrile and about 1.0 to about 2.0 weightpercent other organic compounds including oxazole, allyl alcohol,acetone and propionitrile. Suitable processes for the catalyticammoxidation of propylene to acrylonitrile are disclosed in U.S. Pat.Nos. 5,093,299; 4,863,891; 4,767,878; and 4,503,001, all of which arehereby incorporated by reference in their entirety.

After the ammoxidation reaction, it is necessary to isolate the desiredacrylonitrile, acetonitrile and HCN products from the reactor effluent.In a typical process, the effluent is treated to remove residualammonia. The remaining mixture is suitably treated by a number ofdistillation processes to isolate the desired acrylonitrile. During thispurification process, a product stream rich in acetonitrile is produced.It is this stream rich in acetonitrile that is highly suitable for usein the method of this invention to make purified acetonitrile. Such anacetonitrile rich stream may contain about 30 to about 78 weight percentacetonitrile, about 20 to about 68 weight percent water, about 0.01 toabout 10 weight percent HCN, and about 1.0 to about 5.0 weight percentof other organics such as, for example, oxazole, allyl alcohol, acetone,and acrylonitrile. Processes for the purification of acrylonitrile are,for example, set forth in U.S. Pat. Nos. 4,234,501; 3,885,928;3,352,764; 3,198,750; and 3,044,966, which are hereby incorporated byreference in their entirety.

In addition to purifying the crude acetonitrile recovered during themanufacture of acrylonitrile by the ammoxidation of propylene or otherhydrocarbons, the method of this invention can be used to manufacturepurified acetonitrile from recycled streams or waste streams containingacetonitrile. Such recycle streams, for example, can come from someother chemical process where acetonitrile is used as a solvent or as astarting material for the manufacture of other chemical compositions.Such recycle streams are also produced by high pressure liquidchromatography procedures. The acetonitrile, usually in combination withone or more cosolvents such as water or an alcohol such as methanol,ethanol or isopropyl alcohol, is used as the mobile phase in highperformance liquid chromatography. Such procedures produce a wastestream of acetonitrile contaminated with low amounts of various chemicalcompounds, typically much less than 0.1 weight percent, but containingrelatively large amounts, typically about 10 to about 80 weight percent,of one or more cosolvents used in the chromatographic procedure. Thus,as used herein, recycle acetonitrile means a source of acetonitrile thathas been recovered from at least one chemical, analytical or otherprocedure and which contains at least one impurity. Typical impuritiesmay include one or more of, for example, water, methanol, ethanol,isopropanol, allyl alcohol, isopropyl acetate, one or more heptanes,trimethyl silanol, hexamethyl disiloxane, benzene and tetrahydrofuran.The amount of the impurity or impurities is generally about 0.01 weightpercent to about 80 weight percent based on the weight of the mixture,however, the amount of impurities in the recycle acetonitrile is usuallyless than about 25 weight percent with no single component greater thanabout 10 weight percent, based on the weight of the mixture.

In the preferred process of this invention, the crude acetonitrile isfirst distilled in a first distillation column at below atmosphericpressure to remove light components and heavy components, typicallywater, by removing an acetonitrile-containing first side draw stream forfurther processing. By side draw it is meant that theacetonitrile-containing stream is taken from a point between the top andthe bottom of the column. Such a side draw can be taken at a locationbelow the location where the feed enters the column if the feed has arelatively high level of light, lower boiling impurities, or it can betaken at a point above the location where the feed enters the column ifthe feed has a relatively high level of heavy impurities. Preferably,the side draw is located in the middle 80 percent of the column, morepreferably in the middle 66 percent of the column.

The first side draw stream is directed to a second distillation columnwhere it is again distilled. A second side draw stream of purifiedacetonitrile is removed from the second column. Prior to the seconddistillation, the first side draw stream can be treated to removeimpurities which were not removed in the first distillation. Forexample, a chemical treatment can be employed to eliminate one or moreimpurities. When the crude acetonitrile being treated by the method ofthis invention is acetonitrile produced as a byproduct from themanufacture of acrylonitrile by the ammoxidation of propylene, the crudeacetonitrile stream, as mentioned above, usually contains hydrogencyanide (HCN), for example, about 0.01 up to about 10 weight percentHCN. If the crude acetonitrile stream contains HCN, a portion of thisHCN can be removed from the acetonitrile stream as a light end in thefirst distillation column. Nevertheless, some of the HCN may remain inthe first side draw. The HCN in the first side draw can be removedconveniently by any suitable means for removing HCN from an aqueousacetonitrile feed stream. For example, it can be treated with a basesuch as sodium hydroxide, with an aldehyde such as formaldehyde, withozone, or iron salts. However, since a subsequent step in the processpreferably does not utilize a distillation column having a bottomfraction containing water, it is preferable not to use a reagent forremoving the HCN which either forms or has a salt as a byproduct. As analternative, the crude acetonitrile can be treated prior to the firstdistillation column by a digestion process such as that described inU.S. Pat. No. 4,328,075, which is hereby incorporated by reference inits entirety. In that digestion process, the crude acetonitrile streamcontaining the HCN is reacted with a solution of caustic, for examplesodium hydroxide, and an aldehyde, for example formaldehyde, to removethe HCN. As another alternative, the digester as disclosed in U.S. Pat.No. 4,328,075 can be used after the first distillation of this inventionif an additional distillation column, either atmospheric orsubatmospheric, to remove the reagents used by or products formed insuch a digestion step. This would be particularly useful where there isan appreciable amount of HCN or acrylonitrile present in the firstdistillation column.

The use of a vacuum column as a first column in accordance with thisinvention unexpectedly reduces hydrogen cyanide polymerization in thefirst column. The reduction in polymerized HCN provides for reducedfouling of the upper portion of the column and overhead apparatus thusincreasing the time the column can be used before removal of thepolymerized HCN is required. The use of a vacuum column also decreasesthe amounts of heavy components carried downstream in the purificationprocess. This results in a superior acetonitrile product particularlybecause the resulting product has fewer components that absorb in theultraviolet range. It also reduces the amount of water as early aspossible in the purification process thereby reducing the amount ofmaterial to be recycled.

The process of this invention for the purification of crude acetonitrilewill be described in greater detail using crude acetonitrile obtainedfrom the manufacture of acrylonitrile by the ammoxidation of propyleneas described hereinabove. However, it is to be understood that usingsuch crude acetonitrile is only an embodiment of the present inventionand the processing conditions, processing steps, equipment and otherelements of this embodiment can be used for the purification of othersources of impure acetonitrile including recycle or waste acetonitrile,or mixtures of recycle or waste acetonitrile with acetonitrile obtainedfrom other sources such as crude acetonitrile obtained as a byproductfrom the manufacture of acrylonitrile by the ammoxidation of propyleneor other hydrocarbons.

Crude acetonitrile in the form of a mixture comprising about 30 to about78 weight percent acetonitrile, about 20 to about 68 weight percentwater, about 0.01 to about 10 weight percent HCN and about 0.1 to about5 weight percent other organic compounds such as oxazole, allyl alcohol,acetone, and acrylonitrile is distilled in a first fractionation columnat a top column pressure suitably about 1.5 to about 14 psia, preferablyabout 2.5 to about 10 psia, and most preferably about 3.5 to about 5.0psia. The distillation is conducted so that a lower, preferably bottomstream is withdrawn containing the heavy components in the crudeacetonitrile, and an upper, preferably overhead stream containing thelight components. Heavy components are those components having a boilingpoint greater than acetonitrile or the acetonitrile-water azeotrope atthe pressure used to operate the column. Water is the major heavycomponent. Light components are those components having a boiling pointlower than the boiling point of acetonitrile or the acetonitrile-waterazeotrope at the pressure used for the distillation. HCN is a lightcomponent as well as, for example, oxazole, acrolein, and acrylonitrile.A side stream or side draw is removed from the distillation column.Depending on the temperature and pressure selected for the firstdistillation, as well as the location on the column where the sidestream is taken, this side draw comprises acetonitrile and water andother impurities, however, it is rich in acetonitrile compared to thecrude acetonitrile stream. For example, the distillation is conducted sothat the azeotropic side draw stream suitably comprises about 82 toabout 90 weight percent acetonitrile and about 8 to about 16 weightpercent water, preferably about 86 to about 89 weight percentacetonitrile and about 9 to about 17 weight percent water. If the sidedraw is taken at a point on the column above where the feed is added,the side draw stream is preferably taken as a liquid because the vaporphase is richer in light components. If the side draw is taken below thefeed, the side draw stream is preferably taken as a gas because theliquid is richer in heavy components.

The first distillation column preferably has a reflux loop located atthe upper portion of the column at a point above where the first sidedraw is located, preferably at the top of the column. Thus, a portion orthe upper fraction of the column, preferably the top fraction, isreturned to the column as reflux. The reflux ratio, which is the weightratio of the amount of upper or top fraction returned to the columndivided by the amount of overhead or upper fraction of the columnremoved from the column, is suitably about 1 to about 50, morepreferably about 20 to about 30. The upper or overhead fraction notreturned as reflux can be condensed. At least a portion of thecondensate can be directed back to the acrylonitrile purificationprocess as a recycle stream. Preferably, it is recycled to one of theacrylonitrile product distillation columns used for isolatingacrylonitrile. Most preferably, it is the column which can be used tosupply the crude acetonitrile stream useful as a feed material in themethod of this invention. The reflux is preferably returned to the firstdistillation column at a point above the first side draw. Mostpreferably, at the top of the column. The portion of the upper oroverhead fraction that is not condensed is preferably disposed of by,for example, incineration.

The first distillation column may have packing or plates to assist withthe distillation of the crude acetonitrile. If it has plates, the platescan be sieve, trays or bubble cap and the like. The number oftheoretical plates is preferably about 25 to about 40.

In a preferred embodiment, water, in addition to the water containedwith the crude acetonitrile, can be added to the first distillationcolumn. The water, if added, is preferably added to the upper portion ofthe column above a point where the side draw is taken and below wherethe upper or top fraction is removed. The water is added to assistfurther with the distillation and provide for an extractive distillationof the impurities in the crude acetonitrile such as acrylonitrile andmethyl acetate, if present, which is removed in the overheads. Theamount of water added is an amount that provides for the purification ofthe crude acetonitrile to the desired level. However, generally, theamount of water added is such that the weight ratio of water added toacetonitrile present in the crude acetonitrile is about 0.1:1 to about10:1, and more preferably about 0.2:1 to about 5:1.

The first side draw stream may also comprise smaller amounts ofimpurities such as HCN which were not removed as either lights orheavies. For example, as described above, it may contain about 0.01 toabout 0.5 weight percent HCN, or about 0.05 to about 0.1 weight percentHCN. These impurities, particularly the HCN, are preferably removed, forexample, by subjecting the acetonitrile stream to a treatment with achemical reagent or reagents as described above that can react with andremove the HCN from the stream such as an aldehyde like formaldehyde, acaustic such as sodium hydroxide or one or more iron salts.

The first side draw stream, optionally after subjecting it to thedigestion process described above or to some other process or processesto remove residual impurities, is directed to a second distillationcolumn to be purified further. The first side draw stream is distilledin the second fractionation column at a top pressure of about 30 toabout 120 psia, preferably about 60 to about 100 psia, and mostpreferably about 85 to about 90 psia. The distillation is conducted sothat a lower, preferably bottom stream is withdrawn containing the heavycomponents, and an upper, preferably top stream is withdrawn containingthe light component. Heavy components are those components having aboiling point higher than acetonitrile. Light components include thecomponents having a boiling point lower than the boiling point ofacetonitrile and azeotropes such as the water-acetonitrile azeotrope. Aside stream or side draw is removed from the second distillation column.Preferably, the side draw is located in the middle 80 percent of thecolumn, more preferably in the middle 66 percent of the column. Thissecond draw stream is the purified acetonitrile according to the methodof this invention. The product side draw can be a liquid or vapor taken,preferably, below the feed stream entry point on the column.

The second distillation preferably has a reflux loop located at theupper portion of the column at a point above the second side draw,preferably at the top of the column. Thus, at least a portion or theupper fraction of the column, preferably the top fraction, is returnedto the column as reflux. The reflux ratio for the second distillationcolumn, which is the weight ratio of the amount of upper or top fractionreturned to the column divided by the amount of total overhead drawremoved from the column, is suitably greater than about 3, preferablyabout 3 to about 10, more preferably about 5 to about 8. The upper ortop fraction can be withdrawn as a liquid or combination of liquid andvapor. The liquid draw is preferably directed as a feed to the firstcolumn, i.e., the light ends drying column, preferably above the firstdraw point. The vapor, if present, is preferably recycled to the secondcolumn preferably by mixing it with the side draw from the first column.The reflux in the second distillation column is preferably returned tothe second distillation column at a point above the second draw.Preferably, the second distillation column has packing or plates toassist with the distillation of the acetonitrile. If it has plates, theplates can be sieve, trays or bubble cap and the like. The number oftheoretical plates is preferably about 20 to about 30. A purifiedacetonitrile product is withdrawn from the second column as a side drawsuitably below the point where the feed enters the second column. Thepurified acetonitrile is preferably at least about 99 weight percentpure, more preferably at least about 99.8 weight percent pure, and mostpreferably at least about 99.99 weight percent pure. If desired, thisacetonitrile stream can be subjected to further purification to remove,for example, any minor impurities that may, for example, absorb in theultraviolet.

DETAILED DESCRIPTION OF THE FIGURE

A preferred embodiment of the present invention will now be describedwith reference to the attached FIGURE.

Acetonitrile feed, either crude acetonitrile, recycle acetonitrile or acombination thereof is fed via inlet line 9 and water via inlet line 8,or optionally via line 13, is fed to the first fractional distillationcolumn, also called the light ends drying column, 10 wherein theacetonitrile feed and water are distilled at a pressure of about 2.5 toabout 5.0 psia. The distillation is conducted at a rate so as to allowmost and preferably all of any light impurities in the acetonitrile feedto be removed through the light end overhead line 14. The impuritiespresent are extractively distilled by the water fed to column 10. Inaddition, a stream comprising acetonitrile containing a small amount ofheavy impurities is also charged into the light ends drying column 10via line 36 from product column 30. An acetonitrile-water azeotrope richin the lights from the product column 30 enters the light ends columnfeed via line 34. The light impurities (e.g. isopropyl acetate,acrylonitrile, oxazole) in the waste solvent or crude acetonitrile feed,either due to their low boiling points or aided by extractivedistillation in the presence of water, are also withdrawn from column 10as a vapor draw in line 14 and partly refluxed back into the upperregion of column 10 via reflux condenser 15 and reflux line 13.Preferably, the reflux ratio is about 25:1. Unrecovered overheads areremoved through line 14 and transported to vent scrubbers or condensers(not shown) for waste treatment. Water is recovered from the bottom oflight ends column 10 and discharged via line 15 to waste treatment withpartial recycle through reboiler 11. A first acetonitrile/waterazeotrope containing about 88% acetonitrile, about 10% water, about 1000ppm HCN, less than 10 ppm volatile lights and any heavy organics isrecovered via line 16 as a vapor side draw, condensed in condenser 12and transported via line 17 to an optional treatment unit 20 for theremoval of any HCN or acrylonitrile, if present. Remainder of water andmost of the heavies leave column 10 through line 15 and are directed towaste disposal.

In one embodiment, a liquid side draw can be taken if most of theimpurities were light impurities. In another configuration, the locationof the side draw could be below the feed tray location.

If there are no significant impurities such as HCN and acrylonitrile tobe removed after the first distillation, no treatment unit 20 isrequired. Alternatively, an HCN removal unit such as the digesterdescribed in U.S. Pat. No. 4,328,075, can be used to remove HCN beforecrude acetonitrile feed enters the first distillation column.

The HCN-free acetonitrile and water mixture passing out of treatmentunit 20 is charged via line 22 into a product column 30 and is distilledat high pressure, e.g. 50 psia, into top, middle and bottom fractions. Afraction comprising acetonitrile containing heavy impurities iswithdrawn from the bottom of product column 30 via line 301 intoreboiler 31 for partial recycling to column 30 via line 37 and lightends drying column 10 via line 36. A second acetonitrile/water azeotropeis withdrawn from the top of product column 30 and condensed andrecycled as reflux back to the top of product column 30 via lines 33,331 and reflux condenser 332. Part of the condensed liquid can berecycled back to column 10 through line 38. The uncondensed vaporscontinue via line 34 to the azeotrope condenser 12 where they are mixedwith the first azeotrope from line 16. Alternatively, these uncondensedvapors may be rerouted to light ends drying column 10 via lines 34 and341. Preferably, the reflux ratio as defined above is about 5:1 for thisstep of the process. Because product column 30 is operated at highpressure, water in the second acetonitrile/water azeotrope charged intoproduct column 30 is recovered in the overhead stream of product column30, i.e. the second acetonitrile/water azeotrope, leaving high purityacetonitrile in the product column. This high purity acetonitrile (99.99wt % acetonitrile) is drawn off column 30 as a sidestream via line 39(this stream may be a vapor or liquid, preferably a vapor), and, aftercooling in heat exchanger 321, is discharged via line 56 into anoptional resin treatment bed such as that described in EP 890572 A1.

The above description is not intended to be exhaustive or limiting as tothe description of the present invention, but merely as illustrative ofthe practice of the process of the present invention. It is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description.Accordingly, it is intended to embrace all such alternatives,modifications and variations as fall within the spirit and broad scopeof the appended claims.

EXAMPLE

A first distillation column containing 60 actual trays (50% efficiency)is equipped with an overhead condenser, and a reboiler. A seconddistillation column containing 57 actual trays (50% efficiency) isequipped with an overhead condenser and a reboiler. The operatingpressures for the first column and the second are 3.0-9.0 Psia. and85-90 Psia., respectively. Solvent water at a rate of about 1 gallon perminute (GPM) at 90° F. was introduced at the top tray of the firstcolumn. A crude acetonitrile stream (9.0 GPM) having the composition of66 wt % acetonitrile, 28.8 wt % water, 2.5 wt % HCN, 800 part permillion by weight (PPM) acrylonitrile, 1.3 wt % propionitrile, 1.2 wt %oxazole, and other trace impurities is fed at 75° F. to the firstdistillation column (operated under reduced pressure stated above) attray 40 at a rate of 9.0 GPM. The reflux ratio for the first column ismaintained at 30.

A liquid side-draw of 3970 lb/hr is withdrawn from tray 24 of the firstdistillation column and introduced on tray 16 to the second column(operated at the elevated pressure stated above). The reflux ratio forthe first column is maintained at 6.8. A vapor side-draw of 1938 lb/hr,taken from tray 38 of the second column, is 99.95 wt % acetonitrile,with less than 600 PPM propionitrile and less than 5 PPM oxazole.

The above description is not intended to be exhaustive or limiting as tothe description of the present invention, but merely as illustrative ofthe practice of the process of the present invention. It is evident thatmany alternatives, modifications and variations will be apparent tothose of skill in the art.

U.S. Provisional Application No. 60/218,865 filed Jul. 18, 2000 ishereby incorporated by reference in its entirety.

That which is claimed is:
 1. A method for the purification of crudeacetonitrile comprising distilling the crude acetonitrile in a firstfractional distillation column at below atmospheric pressure,withdrawing a first side draw fraction comprising acetonitrile,distilling the first side draw fraction in a second fractionaldistillation column at super atmospheric pressure, and withdrawing fromthe second distillation a second side draw fraction comprising purifiedacetonitrile.
 2. The method of claim 1 wherein the crude acetonitrilecomprises crude acetonitrile produced by the catalytic ammoxidation ofpropylene or propane.
 3. The method of claim 1 wherein the crudeacetonitrile comprises waste acetonitrile.
 4. The method of claim 1wherein the crude acetonitrile comprises about 38 to 78 weight percentacetonitrile, about 20 to about 68 weight percent water, and about 0.01to about 10 weight percent HCN.
 5. The method of claim 1 wherein thepressure in the first fractional distillation column is about 1.5 toabout 14 psia.
 6. The method of claim 1 wherein the pressure in thesecond fractional distillation column is about 30 to about 120 psia. 7.The method of claim 1 wherein the crude acetonitrile comprises water andwherein the first side draw fraction comprises about 82 to about 90weight percent acetonitrile and about 8 to about 16 weight percentwater.
 8. The method of claim 1 wherein the crude acetonitrile comprisesHCN and wherein the first side draw fraction further comprises HCN andis treated to reduce the amount of HCN contained therein prior todistilling the first side draw fraction in the second distillationcolumn.
 9. The method of claim 1 wherein the crude acetonitrilecomprises HCN and is treated to remove HCN prior to distillation in thefirst fractional distillation column.
 10. The method of claim 1 whereinthe purified acetonitrile is at least 99 weight percent pure.
 11. Themethod of claim 1 wherein water is added to the first fractionaldistillation column.
 12. The method of claim 11 wherein the water isadded to the first fractional distillation column above where the firstside draw fraction is withdrawn.
 13. The method of claim 11 wherein theamount of water added is such that the weight ratio of water added toacetonitrile present in the crude acetonitrile is about 0.1:1 to about10:1.